Electromagnetic wave transmissive metal walls utilizing projecting dielectric rods



March 21, 1967 .T. FRIIS 3,310,808

ELECTROMAGNETIC E TRANSMISSIVE METAL WALLS UTILIZING PROJEGTIN IELECTRIC RODS Filed Dec. 1963 FIG. 1

G6) Q w 3. LL LI 2 FIG. 5.

United States Patent 3,310,808 ELECTROMAGNETIC WAVE TRANSMISSIVE METAL WALLS UTILIZING PROJECTING DIELECTRIC RODS Harald T. Friis, Rumson, N.J., assignor to Hazeltlne Research, Inc., a corporation of Illinois Filed Dec. 30, 1963, Ser. No. 334,542 2 Claims. (Cl. 343-872) This invention relates to walls for providing protection from extreme environmental stresses, while transmitting electromagnetic waves. More specifically, this invention relates to walls using a metal shell with projecting dielectric rods. Electromagnetic Wave Transmissive Metal Walls Utilizing Dielectric Rods are covered more broadly in a copending application of that title of H. A. Wheeler, filed Dec. 30, 1963, Ser. No. 334,541.

A microwave antenna may require protection from environmental factors such as wind, over-pressure, precipitation and extremes of temperature. This protection can be furnished by an enclosure which must be essentially transparent to the electromagnetic waves radiated by the antenna. One form of typical prior art enclosures are radomes which are made of a wall of dielectric material, such as fibre glass, of either a solid or a layered cross section. However, in application having a very severe environment, especially where very high over-pressures may occur, a stronger and more durable structure is required to protect an antenna.

The object of the invention therefore is to provide new and improved walls capable of providing protection from severe environmental stresses.

In accordance with the invention a wall, for resisting environmental stresses while transmitting electromagnetic waves, comprises a metal shell having an array of holes of circular cross-section and arranged in a regular array with a minimum center-to-center spacing in the range of from one-half to one times the operating wavelength and each of the holes has an identical diameter of approximately two-thirds times the spacing. The wall also includes a plurality of dielectric rods supported in the holes so as to go through the shell and project on both sides, the ends of the rods being shaped to provide a transition between free space and the dielectric-filled waveguides formed by the combination of the holes and the rods for electromagnetic waves; the wall being so constructed and arranged as to provide protection from extreme environmental stresses while transmitting electromagnetic waves substantially without distortion.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing:

FIG. 1 shows a general view of a complete radome utilizing a wall constructed in accordance with the invention, and

FIGS. 2 and 3 are views of a section of the wall of the FIG. 1 radome showing greater detail.

Referring to FIGS. 1, 2 and 3, there is shown one form of wall constructed in accordance with the invention, more particularly there is shown a radome for encasing a microwave antenna. FIGS. 2 and 3 are enlarged views of the portion of the wall of the FIG. 1 radome within the dotted rectangle. As shown, the wall includes a metal shell having an array of holes. The shell 10 may be constructed of steel, aluminum or other desired metal. The wall, as shown, also includes a plurality of dielectric rods 12 which are supported in the holes in the metal shell 10. These rods 12 go through the shell 10 3,310,808 Patented Mar. 21, 1967 and project on both sides as shown. The ends of the rods are shaped to provide a transition, between free space and the dielectric-filled waveguides formed by the combination of the holes and rods 12, for electromagnetic waves. It is considered particularly desirable to construct the rods 12 of quartz where great extremes of heat are to be encountered, but any suitable dielectric may be used. It will .be understood that the FIG. 1 radome is completely covered by a regular array of rods 12 supported in holes, even though only a small area of rods is actually shown. Also, the FIG. 3 view should exhibit a small amount of curvature, however, if the radome of FIG. 1 were very large in diameter, the small area of the wall shown in FIGS. 2 and 3 would be essentially flat as illustrated.

In operation, a wave originating from the left and incident on the section of the wall shown in FIG. 3 is captured by the left-hand projections of the rods 12 and then propagated through the dielectric-filled waveguides formed by the holes in the shell 10 and the dielectric rods 12 inserted therein. Finally the wave is re-radiated by the right-hand ends of the rods 12 and the wave then continues to the right. If the rods are spaced closely enough in comparison to a wavelength, the uniformity of the incident wave is essentially maintained in the reradiated wave. If the rod diameter, dielectric constant, and shape of the projecting region of the rods 12 are properly designed there is very little reflection at either side. Therefore, nearly all of the incident wave is transmitted through the wall substantially without distortion. A practical wall will normally be required to operate over a band of frequencies. In this case, the average operating Wavelength can be used as an approximation in design of the wall. Design of walls in accordance with the invention can be carried out using established principles of antenna and waveguide design once the basic concepts of the invention are understood.

Included in FIGS. 2 and 3 are actual dimensions in inches, of a portion of a wall which was actually built and tested, and found to yield nearly complete transmission of an incident wave of normal incidence over a wide band of frequencies centered about nine kilomegacycles. The dimensions shown are merely for purposes of illustration of one particular form of wall in accordance with the invention. In the example of FIGS. 2 and 3, the shell 10 was constructed of steel and the rods 12 were right circular cylinders of quartz (fused silica) with square cut ends as shown. The rods were arranged in a square-type array as shown in FIG. 2.

It should be understood that in walls constructed in accordance with the present invention, each side of the wall is independently designed to pass a desired wave and while inside the shell 10 Waves simply propagate along a dielectric-filled circular waveguide. Since this is true, there is no inherent limitation on the thickness of the metal shell 10 of the wall. Thus, the metal shell 10 could be constructed of steel many inches thick, if desired, and an extremely strong structure would be obtained. Since the dielectric rods 12 are not relied on for strength, the dielectric material can be chosen to withstand other factors of the environment. If quartz (fused silica) is used, the wall would be resistant to extremely high temperatures and other environmental factors.

Ability to withstand extreme stresses is also enhanced in walls constructed in accordance with the invention by the fact that such Walls can utilize holes of size and spacing such that the volume of metal exceeds the volume represented by the holes. By considering a unit section such as enclosed by dotted rectangle 14 in FIG. 2, it will be obvious that the value of metal included exceeds the volume represented by the portions of the holes included.

Walls constructed in accordance with the invention may typically include holes with a minimum center-to-center spacing of from /2 to 1 times the operating wavelength and a hole diameter of approximately /a times said spacing.

Compared with other designs capable of similar performance the invention as described above facilitates economical construction. Dielectric rods 12 can easily be inserted in ordinary circular holes drilled in the metal shell and then cemented or otherwise held in place. Each rod is simple in form and identical with the other, so that mass production of the rods is readily possible. A wide variety of rod spacings, cross sections and end shapes can be utilized in difierent applications.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein Without departing from the invention and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A wall, for resisting environmental stresses while transmitting electromagnetic waves, comprising:

a metal shell having an array of holes of circular crosssection and arranged in a regular array with a minimum center-to-center spacing in the range of from one-half to one times the operating wavelength and each of said holes has an identical diameter of approximately two-thirds times said spacing;

and a plurality of dielectric rods supported in said holes so as to go through the shell and project on both sides, the ends of said rods being shaped to provide a transition between free space and the dielectric-filled waveguides formed by the combination of said holes and said rods for electromagnetic waves;

the wall being so constructed and arranged as to provide protection from extreme environmental stresses while transmitting electromagnetic waves substantially without distortion.

2. A wall for encasing a microwave antenna comprisa steel shell having a regular square type array of circular holes which are approximately 0.38 times the average operating wavelength in diameter and which have a center-to-center spacing of approximately 0.58 times the average operating wavelength;

and a plurality of cylindrical quartz rods, with square cut ends, bonded into said holes so as to go through the shell and project approximately 0.23 times the average operating wavelength on each side of the shell.

8/1952 Affel.

4/1953 Southworth 343-909 X 0 HERMAN KARL SAALBACH, Primary Examiner.

P. L. GENSLER, Assistant Examiner. 

1. A WALL, FOR RESISTING ENVIRONMENTAL STRESSES WHILE TRANSMITTING ELECTROMAGNETIC WAVES, COMPRISING: A METAL SHELL HAVING AN ARRAY OF HOLES OF CIRCULAR CROSSSECTION AND ARRANGED IN A REGULAR ARRAY WITH A MINIMUM CENTER-TO-CENTER SPACING IN THE RANGE OF FROM ONE-HALF TO ONE TIMES THE OPERATING WAVELENGTH AND EACH OF SAID HOLES HAS AN IDENTICAL DIAMETER OF APPROXIMATELY TWO-THIRDS TIMES SAID SPACING; AND A PLURALITY OF DIELECTRIC RODS SUPPORTED IN SAID HOLES SO AS TO GO THROUGH THE SHELL AND PROJECT ON BOTH SIDES, THE ENDS OF SAID RODS BEING SHAPED TO PROVIDE A TRANSITION BETWEEN FREE SPACE AND THE DIELECTRIC-FILLED WAVEGUIDES FORMED BY THE COMBINATION OF SAID HOLES AND SAID RODS FOR ELECTROMAGNETIC WAVES; THE WALL BEING SO CONSTRUCTED AND ARRANGED AS TO PROVIDE PROTECTION FROM EXTREME ENVIRONMENTAL STRESSES WHILE TRANSMITTING ELECTROMAGNETIC WAVES SUBSTANTIALLY WITHOUT DISTORTION. 